1. Field of the Invention
The present invention relates to an antenna selecting diversity receiving apparatus suitable for use in a radio communication system of a time-division multiplex (hereinafter called "TDM") type using a linear modulating wave.
2. Description of the Related Art
FIG. 1 is a block diagram showing a control circuit employed in an antenna selecting diversity receiving apparatus which is disclosed, for example, in U.S. patent application Ser. No. 126,548 filed Nov. 30, 1987. In the same drawing, numeral 1 is a sample-and-hold circuit used to sample received electric field signals represented in the form of electric-field levels received by respective antennas, (hereinafter called "RSSI") which signals are delivered from an unillustrated radio receiver, and then to hold the sampled level therein.
Numeral 2 is a comparing circuit for comparing the level of one of RSSI signals sampled and held by the sample-and-hold circuit and the level of one of RSSI signals from the radio receiver. In addition, numeral 3 is an antenna switch control circuit comprised of first to third exclusive OR gates 4 through 6 and a D flip-flop 7 for controlling an unillustrated antenna switching circuit based on the result of comparison by the comparing circuit 2 to thereby select an antenna which brings the electric fields into a most-satisfactorily receivable state.
The operation of the control circuit will next be described.
Since land mobile radio communications normally occur over line-of-sight paths between radio mobile stations, and radio waves arrive after they are scattered, diffracted and reflected through a medium, the propagation path thereof is defined in the form of a multiwave propagation path. Thus, a distribution of electromagnetic fields having random standing-wave characteristics is formed by the interference of the multiwaves, and fading is produced because a mobile station runs under the distribution of the electromagnetic fields. As a consequence, the level of electric fields received by one antenna mounted on the mobile station is varied as shown in FIG. 2.
Therefore, there has normally been adopted, in the above-described mobile radio communications, an antenna selecting diversity system of the type wherein a plurality of antennas are prepared so as to select one of the antennas, which can bring electric fields into a most-satisfactorily receivable state at that time. If the driving speed of the mobile station is set to 100 Km/h, the fading frequency at the time of occurrence of fading is on the order of 80 Hz at the band of 900 MHz.
A description will hereinafter be made of the antenna selecting diversity circuit employed in the apparatus shown in FIG. 1. FIG. 3 is a timing chart for describing its operation. Assuming that two time slots, i.e., "A" and "B" are defined in the mobile radio communication system and the time slot "A" out of them is allotted to a corresponding radio receiver as shown in FIG. 3.
First of all, electric-field levels received by two antennas are monitored during a period of the time slot B immediately before the time slot A allotted to the radio receiver. A RSSI signal received by one of the two antennas, which is first detected by the radio receiver, is input to the sample-and-hold circuit 1 so as to be sampled and held therein and to be supplied to an inverting input terminal of the comparing circuit 2. On the other hand, an output signal of the sample-and-hold circuit 1 is input to a non-inverting input terminal of the comparing circuit 2. Thus, if a RSSI signal received by the other of the two is input to the radio receiver, then the comparing circuit 2 compares the corresponding RSSI signal with the RSSI signal which has been sampled and held in the sample-and-hold circuit 1 to thereby supply the result of its comparison to the antenna switching control circuit 3.
The result of its comparison is input to one of the input terminals of a first exclusive OR gate 4 in the antenna switching control circuit 3. A Q output of the D flip-flop 7 whose data port is supplied with the output of the exclusive OR gate 4 is input to the other of the input terminals of the exclusive OR gate 4. In addition, a sample-and-hold signal is input to one of the input terminals of a second exclusive OR gate 5, and the other of the input terminals is set to be a high level (+5 V). Thus, the sample-and-hold signal is inverted by the second exclusive OR gate 5.
The sample-and-hold signal is also input to a clock terminal of the D flip-flop 7. A Q output of the D flip-flop 7 is supplied to one of the input terminals of a third exclusive OR gate 6. In addition, the output of the second exclusive OR gate 5 is supplied to the other of the input terminals of the exclusive OR gate 6. Thus, their exclusive OR is output from the antenna switching control circuit 3 as an antenna switch output.
Accordingly, an antenna switch output signal is newly output based on the result of comparison between the RSSI signals received by the two antennas immediately before the start point of the time slot A, and the antenna switch output at that time, so that an antenna capable of receiving a RSSI signal of a higher level is selected.
FIG. 4 is a diagram for describing the transition of a carrier subjected to minimum phase-shift modulation (hereinafter called "MSK") on a vector plane. The phase only in the carrier on the vector plane is varied and its magnitude is constant in the MSK system as shown in the drawing. Thus, if a comparison is made between RSSI signals received instantly by the two antennas immediately before the start point of the allotted time slot, the selection of an antenna, which can bring electric fields into a most-satisfactorily receivable state, can be realized with high possibility during the period of the allotted time slot.
The antenna selecting diversity system referred to above is normally effective for use in a modulation system with a constant envelope included therein as in the case of phase and frequency modulation systems.
The conventional antenna selecting diversity receiving apparatus is constructed as described above. Therefore, this apparatus is effective in a time-division radio communication system using a modulation method with the constant envelope. However, this apparatus is accompanied by the problem that it cannot be applied to a time-division radio communication system based on a linear modulation scheme using a linear modulating wave wherein the components or the envelope of the signal are also varied as well as a variation in its phase, as in the case of two-phase phase-shift modulation (hereinafter called "BPSK") system, quadruple-phase modulation (hereinafter called "QPSK") system and a 16-value quadrature amplitude modulation (hereinafter called "16 QAM") system, or the like.